In recent years, electronic equipment such as a display device or an image sensor provided with an optical sensor portion utilizing a TFD has been developed.
Patent Document 1 discloses an image sensor in which an optical sensor portion utilizing a TFD and a driving circuit utilizing a thin film transistor (Thin Film Transistor: TFT) are provided on one and the same substrate. Patent Document 2 discloses a display device provided with an optical touch panel. In this display device, an optical sensor portion utilizing a TFD is provided in a display area of a liquid crystal display device, so that an object such as a finger which touches the screen can be detected by utilizing ambient light. In addition, Patent Document 3 discloses a display device provided with an optical touch panel in which an object such as a finger which touches the screen can be detected by utilizing not only ambient light incident on the display panel but also reflected light of light from a backlight.
In the above-described display devices or in the image sensors, the TFD is formed by utilizing a low-temperature poly-silicon film, for example. In this case, since a semiconductor layer (the low-temperature poly-silicon layer) of the TFD is thin, the semiconductor layer of the TFD cannot sufficiently absorb light having relatively longer wavelength (e.g. red light) and there is some possibility that high sensitivity cannot be attained. It is considered that the semiconductor layer of the TFD is made thicker. However, if the semiconductor layer is made thicker, the process for crystallizing an amorphous silicon film by laser crystallization or other means functions as the rate-limiting factor. Accordingly, in view of the productivity, it is difficult to greatly increase the thickness of the semiconductor layer of the TFD.
Therefore, conventionally, in order to improve effective sensitivity of an optical sensor utilizing a TFD, it is necessary to increase the area of the light receiving region by arranging a large number of diodes in parallel on a substrate. However, if such a configuration is applied in a touch panel type display device, a large number of diodes are arranged in the pixel area, thereby significantly deteriorating the display characteristics.
On the contrary, Patent Document 4 suggests that a reflecting film is disposed via an insulating film below the semiconductor layer of the TFD, and that the thickness of the insulating film is optimized so as to increase the reflectance of light having long wavelength (red light). Due to the suggestions, the document describes that it is possible to increase the sensitivity of optical sensor to the red light.